Interinstitutional Plan Quality Assessment of 2 Linac-Based, Single-Isocenter, Multiple Metastasis Radiosurgery Techniques

PURPOSE: Interest and application of stereotactic radiosurgery for multiple brain metastases continue to increase. Various planning systems are available for linear accelerator (linac)–based single-isocenter multiple metastasis radiosurgery. Two of the most advanced systems are BrainLAB Multiple Metastases Elements (MME), a dynamic conformal arc (DCA) approach, and Varian RapidArc (RA), a volumetric modulated arc therapy (VMAT) approach. In this work, we systematically compared plan quality between the 2 techniques. METHODS AND MATERIALS: Thirty patients with 4 to 10 metastases (217 total; median 7.5; V(min) = 0.014 cm(3); V(max) = 17.73 cm(3)) were planned with both Varian RA and MME at 2 different institutions with extensive experience in each respective technique. All plans had a single isocenter and used Varian linac equipped with high-definition multileaf collimator. RA plans used 2 to 4 noncoplanar VMAT arcs with 10 MV flattening filter-free beam. MME plans used 4 to 9 noncoplanar DCAs and 6 MV flattening filter-free beam, (minimum planning target volume [PTV(min]) = 0.49 cm(3); PTV(max) = 27.32 cm(3); PTV(median) = 7.05 cm(3)). Prescriptions were 14 to 24 Gy in a single fraction. Target coverage goal was 99% of volume receiving prescription dose (D99% ≥ 100%). Plans were evaluated by Radiation Therapy Oncology Group/Paddick conformity index (CI) score, 12 Gy volume (V(12Gy)), V(8Gy), V(5Gy), mean brain dose (D(mean)), and beam-on time. RESULTS: Conformity was favorable among RA plans (median: MME CI(RTOG) = 1.38; RA CI(RTOG) = 1.21; P < .0001). V(12Gy) and V(8Gy) were lower for RA plans (median: MME V12 = 23.7 cm(3); RA V12 = 19.2 cm(3); P = .0001; median: MME V(8Gy) = 53.6 cm(3); RA V(8Gy) = 44.1 cm(3); P = .024). V(5Gy) was lower for MME plans (median: MME V(5Gy) = 141.4 cm(3); RA V(5Gy) = 142.8 cm(3); P = .009). Mean brain was lower for MME plans (median: MME D(mean) = 2.57 Gy; RA D(mean) = 2.76 Gy; P < .0001). CONCLUSIONS: For linac-based multiple metastasis stereotactic radiosurgery, RapidArc VMAT facilitates favorable conformity and V(12Gy)/V(8Gy) volume compared with the MME DCA plan. MME planning facilitates reduced dose spill at levels ≤V(5Gy).